If Sonarr could be modified to work the way I have CouchPotato configured, it would solve this issue for me and the OP - I’m in a similar situation being not able to use hard-links, though my array is built on Snapraid instead of Unraid.
The ideal solution for me (and what I have CouchPotato doing), is that when a file has completed downloading, first it is copied to the destination (optionally also renamed). Once that has completed, the original file is then replaced with a symlink to the copy. rTorrent has no issues with seeding from a symlink, and as an added bonus with a somewhat obscure ‘find’ command from a shell I can search for files linking to X and thereby discover what original filenames were before renaming if I need to.